Arming control for servo-adjusted tablet compressing machines

ABSTRACT

Compressing apparatus having die filling means adapted to fill a die with formable material, adjustable means for regulating the amount of formable material received in the die and force applying means for applying compressing force to the material within the die. Means are provided for measuring the force applied to the formable material within the die by the force applying means and producing a control signal which varies in proportion to said measured force which in turn indicates the degree to which the die was filled during a given compression operation. First switching means is provided which is responsive to the control signal and adapted to effect adjustment of the adjustable die filling means to control the weight and, therefore, the amount of formable material in each of the dies and thereby the force imparted to the formable material. A prearming circuit including second switching means is provided which is also responsive to the magnitude of said control signal to prevent the first switching means from being effective to effect adjustment of the adjustable die filling means during intervals of time when the magnitude of the signal is below the predefined threshold value.

3,734,663 May 22, 1973 ARMING CONTROL FOR SERVO- ADJUSTED TABLET COMPRESSING MACHINES Primary Examiner-Robert L. Spicer, Jr. Att0rneyWoodhams, Blanchard and Flynn [5 7] ABSTRACT Compressing apparatus having die filling means adapted to fill a die with formable material, adjustable means for regulating the amount of formable material received in the die and force applying means for applying compressing force to the material within the die. Means are provided for measuring the force applied to the formable material within the die by the force applying means and producing a control signal which varies in proportion to said measured force which in turn indicates the degree to which the die was filled during a given compression operation. First switching means is provided which is responsive to the control signal and adapted to effect adjustment of the adjustable die filling means to control the weight and, therefore, the amount of formable material in each of the dies and thereby the force imparted to the formable material. A prearming circuit including second switching means is provided which is also responsive to the magnitude of said control signal to prevent the first switching means from being effective to effect adjustment of the adjustable die filling means during intervals of time when the magnitude of the signal is below the predefined threshold value.

9 Claims, 1 Drawing Figure [75] Inventor: James P. Holm, Kalamazoo, Mich.

[73] Assignee: The Upjohn Company, Kalamazoo,

Mich.

22 Filed: Nov. 16,1971

[21] Appl. No.: 199,133

[52] U.S. Cl. ..425/147, 425/149, 425/256, 425/352 [51] Int. Cl ..B29b 5/06, B29c 3/06 [58] Field of Search ..425/l47, 149, 256, 425/261, 246, 170, 347, 352; 264/40, 109

[56] References Cited UNITED STATES PATENTS 2,329,287 9/1943 Miller ....425/l47 X 2,568,332 9/1951 Genovese ..425/147 2,822,577 2/1958 Schieser et al..... ....425/256 X 3,255,716 6/1966 Knoechel et al. ....425/l70 X 3,628,901 12/1971 Paulson ..425/l49 52 *I s/ v 64 a 47 sup/w T ARMING CONTROL FOR SERVO-ADJUSTED TABLET COMPRESSING MACHINES FIELD OF THE INVENTION This invention relates to a prearming circuit and more particularly to a prearming circuit in compressing apparatus, such as tablet compressing apparatus. Such apparatus has die filling means, adjustable means for regulating the amount of formable material received in the die and force applying means for supplying compressing force to the material within the die and includes means measuring the force applied to the formable material within the die delivering a signal indicative of said force and further includes prearming means responsive to said signal for preventing adjustment of said adjustable means when the magnitude of the signal is below a predetermined threshold value.

BACKGROUND OF THE INVENTION The making of medicinal tablets by compression of powders, dry or treated, is an old art and generally satisfactory machinery for making such tablets has long been available, such as the machinery illustrated in connection with the control equipment of U. S. Pat. No. 3,255,716. A problem, however, has existed with respect to multistation tablet-making machines of this type in that, while the powder applied to such machines maintains reasonably constant characteristics when measured over a substantial period of time, when measured over short periods of time the characteristics will be subject to variations between relatively dense powders and relatively light and fluffy powders. As a result, whenever a die is to be filled with powder, the weight of the powder is not directly related to actual volume displaced by same. Thus, when a force is applied to the powder, the fluffy powder will compress more than will the more dense material and the weight of the tablet formed from the fluffy powder will be less than the weight of the tablet formed from the more dense material. Thus, it is known as set forth in the abovementioned patent to apply measuring means to such machines for measuring the density of such tablets and appropriately adjusting the feed thereto to maintain a substantially constant density. However, this gives rise to a further problem in that if the supply of powder should end, the known circuitry for controlling the machine is not capable of immediately detecting this fact and, as a result, the adjustment circuit attempts to adjust the machine so that more powder will be added to the dies. However, if no powder is present, the machine continues to adjust itself to its limit and then, when more powder is supplied, the machine has to be shut down and manually adjusted back to between the predefined tolerance limits before the operation thereof can properly be resumed. The machine will, of course, make its own adjustment automatically but the powders are normally valuable and an undesirable amount will be formed into unusable tablets if only the automatic control of the machine is relied upon to reset same to its operating condition before the machine reaches its proper setting.

Accordingly, the objects of the invention include:

1. To provide a prearming circuit which will cause power to be supplied to the control circuit for the die filling means in a tablet making machine which controls the weight of the tablet being produced only when the force applying member is applying a force to the material on which work is to be performed, such as formable material in a die, which is within prescribed limits.

2. To provide a prearming circuit, as aforesaid, which includes an alarm mechanism which is activated if the conditions at the filling station remains at fault for a predetermined interval of time so that the machine operator will become aware of a pending malfunction in the machinery.

3. To provide a prearming circuit, which can be readily adapted for use on existing machinery.

4. To provide a prearming circuit which can be made inexpensively and with conventional components.

Other objects and purposes of the invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawing which illustrates the prearming circuit embodying the invention.

SUMMARY OF THE INVENTION The invention refers to a compressing apparatus for forming tablets by applying a compressive force to formable material within a succession of dies and to control means for controlling the amount of formable material supplied to said dies. Force measuring means measure the force applied to the successive dies and produces a signal which automatically adjusts the amount of powder received within said dies. According to the invention there is also supplied a prearming circuit which broadly includes:

a. interpretive means responsive to the magnitude of the control signal and producing an output signal when the magnitude of the control signal exceeds a predefined threshold value,

b. first switching means responsive to the control signal and adapted to effect adjustment of the adjustable die filling means to thereby adjust the force imparted to the formable material by the force applying means, and

c. second switching means responsive to the magnitude of the output signal to prevent the first switching means from activating the adjustable die filling means during intervals of time when the magnitude of the control signal is below the predefined threshold value.

DETAILED DESCRIPTION Referring now to the drawing, there is shown a fragmentary cross section of one station of a typical multistation tablet making machine 10 in connection with which the invention may be carried out.

Inasmuch as such multistation tablet-making machines are well known and since the essential operating parts of one type thereof is adequately illustrated in the above-mentioned US. Pat. No. 3,255,716 it is unnecessary to set forth the details of such a machine herein but instead only those parts thereof which are essential to an understanding of the present invention will be illustrated and described herein. For further details of such a machine to the extent that same is necessary if at all, attention is invited to said above-mentioned patent.

In this machine, an upper punch 11 and a lower punch 12 form a part of and cooperate with a table having a plurality of dies 13 therein to form a tablet in a suitable and known manner. More specifically, suitable formable material to be tableted, such as medicinal powder or granules, is poured (by conventional means not illustrated) into an opening within the rightwardmost die 13 above the respective lower punch 12. The lower punches 12 are conventional and move leftwardly along the cam 151 with the material to be formed into tablets. The volume of material retained in the dies 13 is determined by the vertical position of the cam 152 and the lower punches 12 operating therewith. Thus, the vertical movement of the cam 152 will effect a control of the volume of the material retained in the dies 13. A scraper 153 slidably engages the upper surface of the table after the lower punches successively ride upon the cam 152.

After the dies 13 have been filled, they move leftwardly to a location where the upper punch is brought down and the lower punch brought up in a known manner to exert pressure onto the powder and compress it into a tablet. The upper punch then retracts and the lower punch is driven upwardly (also by conventional means not illustrated) against the tablet to drive same out of the die to be transferred to suitable collecting means (not shown) by which the tablet is removed from the tablet making machine. Normally, as is well understood, a typical multistation tablet making machine will have as many as 35 of such combinations of upper and lower punches and dies cooperating with each other in the same manner as above-described for parts 11, 12 and 13. Both the upper and lower punches 11 and 12, respectively, are mounted upon the rotary table. The rotary table is caused to rotate past the means for feeding the powder successively into the respective dies and also past means for removing the finished tablets following their ejection from the respective dies.

The vertical position of the cam 152 is controlled by a servo motor 154 which is connected through a gear train indicated generally at 156 to effect a rotation of an adjusting nut 157 which is fixedly secured to a screw 158. The screw 158 is supported for rotation but is held against axial movement by the support 159. The upper end of the screw 158 is threaded in a block 161 which is secured to the cam 152 and which is held against rotation. Thus, when the output shaft 162 of the servo motor 154 is rotated, an adjustment of the vertical height of the cam 152 will be accomplished.

A punch operating mechanism is provided on a frame 16 of the machine 10. The upper punch operative device 14 here comprises a lever arm 17 pivotally secured to a bracket 18 mounted on the frame 16. A pressure roll 19 is rotatably mounted on the lever 17 and is positioned so that the cam surface 21 thereof is aligned with the upper punch 11 as same moves therepast on the table and in the direction of the arrow A. A screw 23 is fixed to the frame 16 and is pivotally engaged with the end of the lever 17.

The lower punch operating mechanism 24 includes a lever 26 pivotally secured to a bracket 27 fixed to the frame 16. A pressure roll 28 is secured to the lever 26 and the cam surface 29 thereof is aligned with the lower punch 12 and engages the lower punch as same moves thereby in the direction of the arrow A. An arm 31 is secured to the end of the lever 26 remote from the bracket 27 and has an adjustment member 33 thereon for controlling the length of the arm 31 and the pressure applied by the punches 11 and 12 to the formable material in the die 13.

To provide a measure of the force applied by the punches 11 or 12 to the formable material, a stress sensitive means is applied to the screw 23. In this particular embodiment, the stress sensitive means is a suitable electrical impedance element 38, said resistance element being firmly cemented to the screw 23, or otherwise mounted thereto in a known manner. The other electrical resistance elements 39, 40 and 41 may, if desired, be secured to the screw 23 in a like manner but not illustrated as further described in said U. S. Pat. No. 3,255,716. As shown, only impedance element 38 is mounted on the screw but this is largely for illustrative convenience and it will be understood that the preferred arrangement is to utilize the strain sensitive mans for all of the impedances 38-41 and to mount all of them on the screw 23 in the manner shown in said patent. The resistance elements 38, 39, 40 and 41 are connected together as a bridge circuit in a known manner so that any change in length of the screw 23 will be detected by the stress sensing resistance elements. In this circuit, the resistance elements 38, 39, 40 and 41 are connected into a strain gauge bridge 42 and d.c. power from a dc. power supply 43 is connected to the input terminals 44 and 45 through the lines 46 and 47, respectively.

The dc. power supply 43 is energized by ac. power supplied to the input terminals 48 and 49 from the lines 51 and 52 which are connected to a conventional source 53 of line voltage. A lamp 54 is connected across the lines 51 and 52 to indicate that the source 53 is either turned on or turned off by the control switch 56.

The output terminals 57 and 58 of the strain gauge bridge 42 are connected through lines 61 and 62, re spectively, to the and input terminals, respectively, of an ac. differential amplifier 63. The input signals to the input terminals of the ac. amplifier 63 are properly terminated by terminating resistors 64 and 66 connected to a ground bus line 67. The ac. amplifier is energized through lines 68 and 69 from a power supply 71, which power supply is energized by power supplied by the source 53 through the lines 51 and 52 to the input terminals 72 and 73, respectively.

The output terminal 76 of the ac. amplifier 63 is connected through a filter circuit 74 which averages the output signals generated by the punches 11 and 12 passing between the pressure rollers 19 and 28 and thence to the anode of a diode 77 and the cathode of the diode 77 is connected through the line 78 to junction point 79 and thence through a resistor 81 to the input terminal A of a meter 82. The terminal B of the meter is connected through a line 83 to the ground bus line 67. The terminals A and B of the meter 82 are connected to a moving coil (not shown) which has a pointer or indicator member 84 secured thereto and movable therewith. The pointer or indicator member 84 defines an armature member which can function to close the contacts of a pair of switches which will be discussed in detail below. The meter 82 is a conventional meter, for example a API No. 503-KD Double Set Point Meter.

A pair of relays 86 and 87 are mounted on the meter 82 as schematically indicated in the drawing. Each relay 86 and 87 comprises a coil 88 and 89, respectively, with one end of each being connected through a common line 91 and the line 52 to the power source 53 and the other end of the coils 88 and 89 being connected through normally open contacts 92 and 93, respectively, through a common line 94 and the line 51 to the source 53. Thus, a swinging movement of the armature 84 between limits defined by the normally open contacts 92 and 93, will serve to control the function of the relays 86 and 87. It is oftentimes desirable to have the contacts 92 and 93 mounted in such a manner that they may be adjusted relative to one another so that the contacts may be spaced either closely together or further apart to decrease or increase the amount of current required through the terminals A and B to energize each relay 86 or 87. An excess voltage suppressor 96 is connected across the terminals A and B and is designed for intermittent operation to protect the meter from excessive voltage.

The lines 51 and 52 from the power source 53 are connected to a primary winding 97 of a transformer 98. The secondary winding 99 of the transformer 98 is connected to the input terminals of a dc. rectifier 101. The negative output terminal 102 of the rectifier 101 is connected to the ground bus line 67. The positive output terminal 103 is connected through a line 104 to the junction point 106 and thence through the coil 107 of a relay 108 to the anode of a silicon controlled rectifier (SCR) 109. The cathode of the SCR 109 is connected through a line 111 to the ground bus line 67. The gate electrode 112 of the SCR 109 is connected through a line 113 to the junction point 79.

A contact of thermodelay relay 114 is connected in series with a sounding device 116 and a SCR 117 between the lines 104 and the ground bus line 67. The thermodelay relay 114 has a heating element 118 connected between the line 104 and a junction point 119 positioned between the sounding device 116 and the anode of the SCR 117. A master time delay relay 121 is'connected in parallel with the thermodelay relay 114 and the sounding device 116 and serves to control the switch 56 controlling the power input from the source to the tablet machine drive motor M. In this particular embodiment, the theremodelay relay 114 becomes activated after a three second delay and the master time relay 121 becomes activated after a 60 second delay as will be explained in detail below, but these time periods are mentioned as illustrative only and not limiting.

Returning now to the servometer 154, power is supplied thereto through the lines 122 and 123 which are connected to the lines 51 and 52, respectively, and the source 53. The direction of rotation of the output shaft 162 of the servomotor 154 is controlled by signals applied to the input terminals 124 and 126 of the servomotor l54. The circuitry which is activated to control the direction of rotation of the output shaft 162 of the servomotor 154 includes a relay contact AR connected between a junction point 127 on the line 104 and a junction point 128. The relay contact AR is controlled by energization of the coil 107 of the relay 108 and serves to prearm the circuitry discussed below.

A relay contact M is connected in series through a junction point 129, lamp 131 and dropping resistor 132 between the junction point 128 and the ground bus line 67. Similarly, a relay contact M is connected in series through a junction point 133, lamp 134 and dropping resistor 136 between the junction point 128 and the ground bus line 67. The relay contacts M and My are each controlled by the relays 86 and 87, respectively, in the meter 82.

The junction point 129 is connected through a switch contact S1 to a line 137 which is in turn connected to the input terminal 124 of the servomotor 154. The junction point 133 is connected through a switch contact S2 to a line 138 which is in turn connected to the input terminal 126 of the servomotor 154. The switches S1 and S2 are each connected to operate together between an automatic control position shown in the drawing and a manual position. When the switch contacts S1 and S2 are in the manual position, a manually operated push button switch 139, connected between the line 104 and the line 137 is used so that the input terminal 124 of the servomotor 37 can be manually controlled thereby. Similarly, a manually operated push button switch 141 is connected between the line 104 and the line 138 so that the input terminal 126 may be manually controlled thereby.

In the event that one of the lines 137 and 138 is energized for too long a period of time in the automatic mode, namely of the order of three seconds or more, a protective alarm system is provided which comprises a diode 142 connected between the junction point 129 and the gate electrode of the SCR 117. Similarly, a diode 144 is connected between the junction point 133 and the gate electrode 143 of the SCR 117. A Zener diode 146 is connected between the gate electrode 143 and the ground bus line 67 in order to limit the magnitude of potential applied to the gate electrode 143.

OPERATION Although the operation of the circuit described above will be understood from the foregoing description by skilled persons, a summary of such description is now given for convenience.

The die 13 and the punches 11 and 12 are moved in the direction of the arrow A which will bring the upper punch against the upper pressure roll 19 and the lower punch 12 against the lower pressure roll 28. The upper pressure roll 19 acts through the lever 17 to change the stress in the screw 23. This in turn affects the stress sensitive unit 38, and such others of the resistance elements 39, 40 and 41 as may be mounted on the side of the screw 23, to change the output of the strain gauge bridge 42. This output signal is presented to the input terminals of the ac differential amplifier 63 which produces an output signal proportional to the stress signal detected by the strain gauge bridge 42. The signal is averaged by the filter circuit 74. When a signal appears at the output terminal 76 of the amplifier 63, a signal is presented through the lines 78 and 113 to the gate electrode 112 of the SCR 109 to turn the SCR on. Thus, the coil 107 of the relay 108 will become energized to close the contacts AR to supply power to the junction point 128.

The signal from the amplifier 63 will also be delivered through the junction point 79 and resistor 81 to the terminal A of the meter 82 to cause a movement of the armature 84 between the limits defined by the contacts 92 and 93. If the force transmitted to the formable material in the die 13 is too low, the armature 84 will be in engagement with the contacts 92 to complete the circuit therebetween so that the relay coil 88 of the relay 86 is energized. Thus, the contact M is closed and power is initially supplied through the line 137 to the input terminal 124 to cause a rotation of the output shaft 162 of the servomotor 154 to increase the amount of material in the dies 13 by effecting a downward movement of the cam 152. As the servomotor 154 lowers the cam 152, so that more material enters the dies, a higher compression on the die 13 will be created by the punches l1 and 12 and a progressively increasing strain is placed on the strain sensing elements 38 of the strain gauge bridge 42 so that the amplifier 63 will produce a progressively increasing output signal to the meter 82. Eventually, the armature 84 will swing away from the contacts 92 to de-energize the coil 88 and cause the contact M, to open to thereby de-energize the signal to the input terminal 124 of the servomotor 37. Thus, the desired amount of pressure will be applied by the punches 11 and 12 on the formable material in the die 13. However, if a pressure is developed in the die 13 which is greater than desired, a greater output signal will appear at the output terminal 76 of the amplifier 63 which will cause the armature 84 to swing to the other side and engage the contacts 93 to effect an energization of the coil 89 of relay 87. This will result in a closing of the contact M to apply a signal on the line 138 to the input terminal 126 of the servomotor 154 to cause the output shaft 162 to rotate in a manner to raise the cam 152 and thereby decrease the pressure applied by the punches 11 and 12 on the formable material in the die 13. Thus, the desired amount of pressure can continue to be applied to the formable material in the die 13 as same moves past the pressure rolls l9 and 28.

If, for example, the pressure applied by the punches 11 and 12 remains too low or too high for too long a period of time, for example three seconds, then the signal applied through one of the diodes 142 or 144 will cause the SCR 117 to be turned on to energize the heater element 118 of the thermodelay relay 114 and after a three second delay, close the contacts of the relay 114 to energize the sounding device 116. This will let the operator know that an abnormality exists within the machine and he can take steps to correct same if it is necessary. If the abnormality remains for a prolonged period of time, in excess of, for example, 60 seconds, the master relay 121 will become energized to open the machine motor control switch 56 to discontinue the supply of power to the tablet machine drive motor M.

It is to be noted that the control circuit involving the contacts M and My do not become supplied with power until the contact AR becomes closed. Thus, as long as the output signal from the output terminal 76 of the amplifier 63 is sufficiently low to prevent SCR 109 from turning on, the input terminals 124 and 126 of the servomotor 154 will not be activated. Thus, when the machine malfunctions to the point where insufficient formable material is placed into the die to enable the punches to compress same, or if the supply of material is blocked or exhausted, the signal output at the output 76 of the amplifier 63 will become low for a prolonged period of time to sound the alarm 116. If the signal output from the amplifier 63 becomes low enough, the SCR 109 will turn off to de-energize the coil 107 of the relay 108 to cause the contact AR to become opened. Thus, the servomotor 154 will not effect a major adjustment of the relative position of the cam 152 when no strain is placed on the strain sensing element 38. Likewise, a high signal for an excessive period of time will first effect a warning and then turn off the tablet machine motor M prior to the development of excessive pressure on the punches.

If desired, the filter circuit 74 could be replaced by high and low threshold amplifiers both connected to the output of the amplifier 63. An appropriate sync signal could be provided, such as from a convenient magnetic pickup source on the machine, for the threshold amplifiers. Counters are connected to the output of the threshold amplifiers and the output signals derived therefrom are fed to a mixer and thence to the gate electrode 112 of the SCR 109. Thus, the SCR 109 would be responsive, as before, to effect a varying condition caused by'variations in the fill-of the dies 13.

Although a particular preferred embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.

The embodiments of the invention in which an exclu sive property or privilege is claimed are defined as follows:

1. in an apparatus having adjustable die filling means and a power supply therefor to effect an operation of said adjustable die filling means and means for measuring the force applied to formable material in said die by force applying means and for producing a control signal which varies in proportion to the magnitude of said measured force, a prearming circuit comprising:

interpretive means responsive to the magnitude of said control signal and producing at least one output signal where the magnitude of said control signal is outside of a predefined range;

first switching means responsive to said control signal and adapted to effect adjustment of said adjustable die filling means to thereby alter the force imparted to said formable material; and

second switching means responsive to said output signal to prevent said first switching means from being effective to adjust said adjustable die filling means during intervals of time when said magnitude of said control signal is below a predefined activating threshold value.

2. A prearming circuit according to claim 1, wherein said first switching means comprises first and second switches adapted to become conductive or nonconductive in response to the magnitude of said control signal, said first switch being conductive and said second switch being nonconductive when said magnitude of said control signal is not greater than a low predefined threshold value and said first switch being nonconductive and said second switch being conductive when said magnitude of said control signal is not less than a high predefined threshold value, said low predefined threshold value and said high predefined threshold value defining therebetween said predefined range.

3. A prearming circuit, according to claim 2, wherein said second switching means includes a silicon controlled rectifier wherein the gate electrode thereof is controlled as a function of the magnitude of said control signal and a relay switch having its winding connected in series with said silicon controlled rectifier is controlled by the conduction of said silicon controlled rectifier when the magnitude of said control signal applied to said gate electrode is in a predetermined relation to said predefined activating threshold value.

4. A prearming circuit according to claim 3, wherein the contact of said relay switch is in series with the conductive path controlled by said first switching means.

5. A prearming circuit according to claim 2 including further switching means in circuit with the power supply for said adjusting means to permit a manual control of the conductive state of said first and second switches and a normally closed switch in circuit with the power supply to the drive motor for said apparatus and time responsive means for opening said last named switch upon appearance of said output signal for a predetermined period of time.

6. A prearming circuit according to claim 1, wherein said output signal comprises a first output signal component when the magnitude of said control signal exceeds a predetermined minimum value and comprises a second output signal component when the magnitude of said control signal exceeds a predetermined maximum value.

7. A prearrning circuit according to claim 2, wherein said predefined activating threshold value is less than said low predefined threshold value.

8. The prearming circuit according to claim 1, wherein said interpretative means produces two output signals, the first thereof being produced when the magnitude of said control signal is below said predetermined range and the second thereof being produced when the magnitude of said control signal is above said predetermined range and wherein further said first switching means is adapted to adjust said die filling means to increase the force imparted in response to said first output signal and to adjust said die filling means to decrease said force imparted in response to said second output signal and wherein said predefined activating threshold value is less than the lowest value in said predefined range.

9. A prearming circuit according to claim 1, wherein said apparatus is a tablet forming machine and wherein said formable material is a pharmaceutical compound and wherein said adjustable die filling means is adapted to control the volume of said pharmaceutical compound utilized to form same into a tablet. 

1. In an apparatus having adjustable die filling means and a power supply therefor to effect an operation of said adjustable die filling means and means for measuring the force applied to formable material in said die by force applying means and for producing a control signal which varies in proportion to the magnitude of said measured force, a prearming circuit comprising: interpretive means responsive to the magnitude of said control signal and producing at least one output signal where the magnitude of said control signal is outside of a predefined range; first switching means responsive to said control signal and adapted to effect adjustment of said adjustable die filling means to thereby alter the force imparted to said formable material; and second switching means responsive to said output signal to prevent said first switching means from being effective to adjust said adjustable die filling means during intervals of time when said magnitude of said control signal is below a predefined activating threshold value.
 2. A prearming circuit according to claim 1, wherein said first switching means comprises first and second switches adapted to become conductive or nonconductive in response to thE magnitude of said control signal, said first switch being conductive and said second switch being nonconductive when said magnitude of said control signal is not greater than a low predefined threshold value and said first switch being non-conductive and said second switch being conductive when said magnitude of said control signal is not less than a high predefined threshold value, said low predefined threshold value and said high predefined threshold value defining therebetween said predefined range.
 3. A prearming circuit, according to claim 2, wherein said second switching means includes a silicon controlled rectifier wherein the gate electrode thereof is controlled as a function of the magnitude of said control signal and a relay switch having its winding connected in series with said silicon controlled rectifier is controlled by the conduction of said silicon controlled rectifier when the magnitude of said control signal applied to said gate electrode is in a predetermined relation to said predefined activating threshold value.
 4. A prearming circuit according to claim 3, wherein the contact of said relay switch is in series with the conductive path controlled by said first switching means.
 5. A prearming circuit according to claim 2 including further switching means in circuit with the power supply for said adjusting means to permit a manual control of the conductive state of said first and second switches and a normally closed switch in circuit with the power supply to the drive motor for said apparatus and time responsive means for opening said last named switch upon appearance of said output signal for a predetermined period of time.
 6. A prearming circuit according to claim 1, wherein said output signal comprises a first output signal component when the magnitude of said control signal exceeds a predetermined minimum value and comprises a second output signal component when the magnitude of said control signal exceeds a predetermined maximum value.
 7. A prearming circuit according to claim 2, wherein said predefined activating threshold value is less than said low predefined threshold value.
 8. The prearming circuit according to claim 1, wherein said interpretative means produces two output signals, the first thereof being produced when the magnitude of said control signal is below said predetermined range and the second thereof being produced when the magnitude of said control signal is above said predetermined range and wherein further said first switching means is adapted to adjust said die filling means to increase the force imparted in response to said first output signal and to adjust said die filling means to decrease said force imparted in response to said second output signal and wherein said predefined activating threshold value is less than the lowest value in said predefined range.
 9. A prearming circuit according to claim 1, wherein said apparatus is a tablet forming machine and wherein said formable material is a pharmaceutical compound and wherein said adjustable die filling means is adapted to control the volume of said pharmaceutical compound utilized to form same into a tablet. 